1. Field of the Invention
The present invention relates to an apparatus for determining presence and absence of degradation of an oxygen concentration sensor for detecting oxygen concentration in exhaust gas.
2. Description of Related Art
Conventionally a technology is known in which fuel injection quantity is adjusted by feedback control so as to equalize an air-fuel ratio, which is calculated from a detection value of an oxygen concentration sensor, to a target air-fuel ratio. The oxygen concentration sensor detects oxygen concentration in exhaust gas emitted from an internal combustion engine. The air-fuel ratio (a ratio between air and fuel in burned air-fuel mixture) is calculated based on the detection value of the oxygen concentration sensor. In such an internal combustion engine, detection accuracy of the oxygen concentration sensor greatly influences emission amount control. Therefore, it is important to determine degradation of the oxygen concentration sensor with sufficient accuracy.
A detection apparatus described in JP4-365950, which performs such a determination of degradation, performs a dither control, which compulsively changes the air-fuel ratio alternately to rich side and to lean side by controlling fuel injection quantity. If a response time from when the dither control is started to when a change appears in the detection value of the oxygen concentration sensor (that is, to when the detection value exceeds a threshold value) is longer than a predetermined time, it is determined that the oxygen concentration sensor is degraded.
However, in this kind of common oxygen concentration sensor, the detection value compulsively changed by the dither control is accompanied by large fluctuation due to noises etc. Therefore, the detection value can momentarily exceed the threshold value due to the noises just after the dither control is started. Accordingly, the above-mentioned response time is shortened, so that no degradation can be determined though the oxygen concentration sensor is actually degraded.